If you don't see a line like this, edit /etc/my.cnf, comment out the option skip-networking:

vi /etc/my.cnf

[...]
#skip-networking
[...]

and restart your MySQL server:

/etc/init.d/mysql restart

To secure the MySQL installation, run:

mysql_secure_installation

Now you will be asked several questions:

server1:~ # mysql_secure_installation

NOTE: RUNNING ALL PARTS OF THIS SCRIPT IS RECOMMENDED FOR ALL MySQL
SERVERS IN PRODUCTION USE! PLEASE READ EACH STEP CAREFULLY!

In order to log into MySQL to secure it, we'll need the current
password for the root user. If you've just installed MySQL, and
you haven't set the root password yet, the password will be blank,
so you should just press enter here.

By default, a MySQL installation has an anonymous user, allowing anyone
to log into MySQL without having to have a user account created for
them. This is intended only for testing, and to make the installation
go a bit smoother. You should remove them before moving into a
production environment.

Remove anonymous users? [Y/n]<-- ENTER ... Success!

Normally, root should only be allowed to connect from 'localhost'. This
ensures that someone cannot guess at the root password from the network.

Disallow root login remotely? [Y/n]<-- ENTER ... Success!

By default, MySQL comes with a database named 'test' that anyone can
access. This is also intended only for testing, and should be removed
before moving into a production environment.

*Please note: You do not have to do this (but it does not hurt ;-)) if you intend to use ISPConfig on your system as ISPConfig does the necessary configuration using procmail recipes. But please go sure to enable Maildir under Management -> Server -> Settings -> EMail in the ISPConfig web interface.